All of the fandiscs I've read are reviewed in this post, since I prefer to give fandiscs even shorter reviews than I usually write. For VNs like Altered Fable which some consider a fandisc and others consider a sequel, I'll decide what I think it is (or equivalently: how long a review it deserves) after reading it myself.

Before you ask, I will definitely read Sharin no Kuni, Yuukyuu no Shounenshoujo and Rui wa Tomo wo Yobu Fan Disc -Asu no Mukou ni Mieru Kaze- at some point. I will also probably read the rest of Sumairu Cubic! someday, and the Liarsoft fandiscs (Round a Go! Go! and Hotch Potch) after I've read more from Liarsoft.

The "scores" I give to fandiscs are:

・Must-Read - Adds something significant enough (and good enough) to the story that it would be fair to say you "missed out" if you didn't read it.
・Good - If you qualify as a "fan" of the original work, you will enjoy it, though you wouldn't be "missing out" if you chose not to read it.
・Okay - Even if you qualify as a "fan" of the original work, you may or may not enjoy it.
・Bad - Even if you qualify as a "fan" of the original work, you will not enjoy it.

I don't bother with polarization scores for fandiscs. Also, since fandiscs often consist of separate chunks you can pick and choose from, either the rating will refer only to the best chunks, or I'll give different chunks different ratings.

Altered FableThis fandisc has four parts: Kagayaku Toki ga Kienu ma ni, Akatsuki Haruka Nari, Muv-Luv Radio, and a trial for Muv-Luv Alternative Total Eclipse
・Kagayaku Toki ga Kienu ma ni is essentially Muv-Luv Extra all over again (it's even the same length), though the production values are even better and almost nothing is straight-up repeated, so if you enjoyed MLE you should have no trouble enjoying this. I read six out of the ten "routes" (as with MLE and MLU, they're more like endings), and imo Kasumi's was the most interesting.
・Akatsuki Haruka Nari is an infamously difficult hex-based strategy game where you control a group of TSFs fighting the BETA. Personally, the complete lack of a tutorial (plus glitchy invincible Lux) kinda killed my enthusiasm for it.
・Muv-Luv Radio is a series of three MLA radio dramas, which were decently amusing.
Score: Okay

・The trial for Total Eclipse, though obviously quite short, felt like exactly what you'd want from an MLA spinoff and was easily enough to get me excited for reading the full version at some point.
Score: Good

Ayu Mayu AlternativeTo confuse the hell out of everyone, this fandisc has two parts: "Ayu Mayu Alternative" and "Muv-Luv Alternative Chronicles". The latter contains two stories: "~Atonement~" and "~Succession~".
Ayu Mayu Alternative is supposedly inspired by the animated comedy shorts called Ayu Mayu Gekijou, but instead it puts Saint Seiya-style superheroes in the MLA world, changes Ayu and Mayu's personalities and takes itself completely seriously. Although age put a lot of effort into it, it's still mediocre at best. This thing simply has no right to exist.
Score: Bad

As for the Chronicles, ~Atonement~ is about Marimo's time in the army before becoming an instructor, and ~Succession~ is about Isumi's sisters after the Sadogashima operation. Both stories are very short, but very well done in all the same ways MLA itself was, and add quite a bit to those characters.
Score: Good

Rewrite Harvest festa!
Kotori and Chihaya's after stories are nice and sweet. Lucia's after story adds a new character and a surprisingly interesting conflict, making it the best part in my opinion. Shizuru, Akane and Kagari's AU stories are quite silly, even random at times, but also pretty funny. Finally, there's a fun RPG minigame called Rewrite Quest, which doesn't take itself seriously at all. All seven parts are fleshed out nicely and clearly had a lot of effort put into them (especially Rewrite Quest). The fandisc also comes with wallpapers and system voices.
Score: Good

Souten no Celenaria Fan Disc
A mildly entertaining side story involving (loli) Mary Shelley visiting the continent. Pretty much everything I said about Celenaria itself applies to this as well.
Score: Okay

Umineko no Naku Koro ni Tsubasa
15 miscellaneous bonus stories plus a postscript. The serious ones are legitimately interesting (if you like minor character backstories and witches being cryptic), and the more numerous comedy ones are hilarious, albeit shamelessly cliche. Story 13 being boring is the sole exception.
Score: Good

Umineko no Naku Koro ni Hane
2 more bonus stories plus postscript. For some reason they're just not as interesting as Tsubasa's were, though I got some laughs out of the 4th wall breaking in the second one.
Score: Okay

Our Confession (booklet that came with Hane)
A rough outline of a potential Umineko episode that Beatrice planned out but never actually used. Includes explicit mention of all the accomplices and most of the tricks she would have used to construct the mysteries. Though it says nothing about the eight episodes we know, it's an invaluable collection of massive hints.
Score: Must-read